Variable frequency multivibrator



Jan. 19, 1954 M. HOLLINGSWORTH 2,655,352

VARIABLE FREQUENCY MULTIVIBRATOR Filed April 2, 1946 s\= fls |5- ig u Negative Terminal Of Source OF Variable Bias Potential INVENTOR LOWELL M. HOLLINGSWORTH ATTORNEY Patented Jan. 19, 1954 VARIABLE FREQUENCY MULTIVIBRATOR Lowell M. Hollingsworth, Cambridge, Mass., as-

signor, by mesne assignments, to the United States of America as represented by the Secretary of War Application April 2, 1946, Serial No. 659,049

3 Claims.

This invention relates in general to electrical circuits and more particularly to multivibrator oscillators.

The principles and applications of the multivibrator type of oscillator are well known in the electronics art. The ability of these circuits to provide periodic waveforms of variable frequency and shape has made them standard devices in many types of radio and electronic apparatus.

In the past, variation of the fundamental operating frequency of such a circuit has been accomplished by alteration of the values of various resistors and capacitors in the circuit. It is often difiicult by such means to provide wide frequency variation in the multivibrator without afiecting the stability of the circuit. In addition, such mechanical tuning becomes highly awkward when the control knob or dial must be placed at any distance from the multivibrator itself.

Various forms of voltage controlled multivibrators have been devised but these suifer from the disadvantage of requiring an extra voltage supply.

Accordingly, it is one object of the present invention to provide a multivibrator, the frequency of which may be controlled.

Another object of the invention is to provide a multivibrator tunable in frequency over a wide range.

Still another object is to provide a multivibrator which is tuned by a variable voltage rather than by mechanical. means.

Briefly, the present invention contemplates a multivibrator in which the vacuum tubes are bypassed by a second set of vacuum tubes, the latter operating in a manner similar to that of variable resistances in shunt with the multivibrator tubes. A variable negative bias applied to the control grids of the shunt tubes controls their efiective resistance.

The principles and operation of the invention will be more apparent to those skilled in the art upon reference to the following specification and claims, and to the drawing in which the single figure represents a schematic diagram of one embodiment of the invention.

Referring now to this figure, vacuum tubes 5 and 6 form a part of a multivibrator circuit, plate I of tube 5 being connected through load resistor 8 to a positive voltage supply. Plate of tube 6 is connected through load resistor II to the same voltage source. Grid 12 of tube is connected to the positive voltage source through resistor l3 and grid M of tube 6 is similarly connected through resistor I5. Anode ID of tube 6 is connected through coupling capacitor l8 to grid l2 of tube 5 and anode l of tube 5 is connected through capacitor I9 to grid it of tube 6. Cathode 20 of tube 5 and cathode 2! of tube 6 are grounded. This combination and connection of circuit elements constitutes a normal positive grid-return multivibrator circuit although the grids may be returned to ground without making the circuit inoperative. However, anode 25 of vacuum tube 25 is connected to anode i of tube 5 and cathode 21 of the former tube is connected to cathode 23 of tube 5. In like fashion, anode 30 of vacuum tube 3! and cathode 32 of the same tube are connected directly to anode I0 and cathode 2|, respectively, of tube 6. Grid 34 of vacuum tube 26 and grid 35 of vacuum tube 3! are tied together, and connected to a source of variable negative bias voltage.

As is well known in the art, operation of a multivibrator includes alternate conduction of the tubes caused by the periodic charging and discharging of the coupling capacitors. During the time when one of the tubes, for example tube 5, is out 01f, the coupling capacitor, in this case capacitor l9, attached to its plate, charges through load resistor 83. However, due to the fact that tube 29 is drawing its plate current through resistor 8, dropping the potential at anode '5, capacitor l9 will not charge to as high a value of voltage as it would if tube 28 were not connected into the circuit. Therefore, during the next portion of the operating cycle when tube 5 is conducting and tube 6 cut off, the depressing efiect of capacitor ii! on grid [4, which decreases exponentially with discharge of the capacitor, will persist for a shorter time than would otherwise be the case, shortening this portion of the cycle. Since the other half of the cycle is similarly affected by the action of tube 35, the net result is an increase in the fundamental frequency of the multivibrator. When either one of the multivibrator tubes is conducting, the voltage drop in its plate load resistor so lowers the plate voltage of the associated shunt tube that the latter is rendered nonconductive. Simple shunt resistors would not be so satisfactory in the circuit since they would draw current continuously.

The circuit is highly adaptable in that the basic multivibrator may be either balanced or unbalanced, either one or both of the multivibrator tubes may be shunted, and the grids of the shunt tubes may be connected to separate varying bias sources, rather than the same bias source.

In an exemplary embodiment of the invention,

obvious to those skilled in the art that changes and modifications may be Inade'therein without exercise of inventive ingenuity? Hence, allis'uqh modifications and adaptations are claimed as may fall fairly within the true spirit and scope of the invention.

What is claimed is:

1. A variable frequency multivibrator comprising a first multi-electrode vacuum tubehaving at least anode, cathode,'and' control electrodes; a first plate load resistor associated with said first vacuum tube, a first grid resistor associated with said first vacuum tube, a second multi-electro de vacuum tube having at least anode, cathode, and control electrodes, a, second plate load resistor associatedwith said second vacuum tube, a secondgridres'istor associated with said second vac uum tube, said grid andplate resistors being connected to a source of positive potential, first capacitor'means coupling the anode of said. first vacuum tube to the c'ojntrol electrode of said sec-' ond vacuum tube, second capacitor means couplingthe anode of said' second "vacuum tube to the control electrode of said first vacuum tube, a third multi-electrode loading vacuum tube having at least anodef'cath'odej and control electrodes, means for electrically connecting the anode of said third vacuum tube to the anode of said first "vacuum tube, means for electrically connecting the cathodeof said thirdvacuum tube to the cathode of said first vacuum tube, a fourth multi-electrode loa'dingf vacuuin tube" having at least anode, cathode, and control electrodes, means for electrically connecting the anode of said fourth vacuum tube to the anode of said second vacuum tube, means for electrically connecting the cathode of said fourth vacuum tube to the cathode of said second vacuum tube, and means for biasing the control electrodes of said third and said; fourth vacuum tubes negatively.

2. Apparatus in accordance with claim 1 wherein addition said negative bias means is variable in output magnitude.

3.'I n combination, a free-running, symmetrical multivibrator including a pair of triode vactubes, the anodes of each of said tubes being respectively coupled to the control grids of the other of said tubes, first and second loading triode vacuum tubes respectively connected in shunt with the respective cathode-anode current paths of the first-named pair of triodes, and means for variably biasing the control grids of saidloading triodes so as to vary the non-conducting periods of said. first-named pair of triodes;

LOWELL M. HOLLINGSWORTH.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,934,322 Osbon Nov. 7, 1933 1,979,484 Mathes Nov. 6,1934 2,389,992 Mayle Nov. 27, 1945v 2,402,989 Dickinson -1- July 2,1946 2,409,229 Smith, Jr., et al. Oct. 15, 1946 2,413,932 Sziklai Jan. 7,1947 2,441,579 Kenyon May 18,1948 2,452,549 Cleeton Nov. 2, 1948.

OTHER REFERENCES Review of Scientific Instruments, November 1937, vo1. 8, pp. 414 to 416, A Vacuum Tube Circuit for Scaling 'Down Counting Rates}? by Stevenson and Getting. 

